Biomimetic Bouligand chiral fibers array enables strong and superelastic ceramic aerogels

Ceramic aerogels are often used when thermal insulation materials are desired; however, they are still plagued by poor mechanical stability under thermal shock. Here, inspired by the dactyl clubs of mantis shrimp found in nature, which form by directed assembly into hierarchical, chiral and Bouligand (twisted plywood) structure exhibiting superior mechanical properties, we present a compositional and structural engineering strategy to develop strong, superelastic and fatigue resistance ceramic aerogels with chiral fibers array resembling Bouligand architecture. Benefiting from the stress dissipation, crack torsion and mechanical reinforcement of micro-/nano-scale Bouligand array, the tensile strength of these aerogels (170.38 MPa) is between one and two orders of magnitude greater than that of state-of-the-art nanofibrous aerogels. In addition, the developed aerogels feature low density and thermal conductivity, good compressive properties with rapid recovery from 80 % strain, and thermal stability up to 1200 °C, making them ideal for thermal insulation applications.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): In this work, strong and superelastic ceramic aerogels were fabricated by utilizing the Bouligand structure.The idea for preparing the material is fresh and interesting as well as the outstanding result.However, the structure and writing style of the paper is a bit difficult for readers to follow.There are some information and variables that are not introduced well for full understanding.
1.In the Abstract, could the author include the density and insulation properties of the developed material since it aims for insulation and aerospace technology? 2. In the Introduction, data should be added to support opinions, e.g., "Ceramic aerogels exhibit an extraordinary combination of low thermal conductivity, high stability in extreme environments,…", "…It has been shown that the transition from longitudinally staggered array to Bouligand chiral array leads to a monotonic decrease in anisotropic mechanical properties…" 3. Could the author introduce some previous research working with the Bouligand structure to illustrate how good it is? 4. What are the advantages of combining both macro-and nano-scale components into ceramic aerogels?Where is the idea come from? 5.The paragraphs after the subheading "Fabrication and Bouligand chiral architecture of BcF-CAs" look a bit long and difficult to understand.Could the author shorten them and make them clearer?In addition, it could be better to have a diagram to depict how BcF-CAs are fabricated from preparing nanofibrous membranes to the final material.6. Leter "ɑ" and "α" are not used consistently in the Figures and the paragraphs even though they have the same meaning.The ϕ symbol should be illustrated in Fig. 1b.7. Temperature and the surface of the material before and after applying the butane blowtorch should be shown in Fig. 1m to support the idea "…the BcF-CA maintained their shape and structure even under such extreme conditions…".8.The author should investigate the effect of AlBSi sol on the mechanical properties of the material to choose 5 wt% as an appropriate concentration.9. Could the author show how to determine the thickness of the bonding layer?10.Could the author give more description for Fig. 2b? 11.Fig. 11d does not look quite obvious.
12. As the maximum tensile stress is around 170.38 MPa, however, the axis value for the stress in Fig .2g used the unit of GPa, which is difficult to see how the stress change over the temperature range.13. what is the micro-fiber content and ratio of fibers mentioned in the manuscript?How to control them when preparing the material?

Response summary:
We sincerely thank the reviewer for the invaluable comments on our manuscript; the insightful comments have played a crucial role in enhancing the overall quality of the work.In response to their suggestions, we have diligently revised the manuscript and double-checked similar deficiencies throughout the manuscript.We earnestly hope that the reviewers find our responses satisfactory and convincing.
Detailed responses to each comment are provided below.

Comments from Reviewer #1:
In this work, strong and superelastic ceramic aerogels were fabricated by utilizing the Bouligand structure.The idea for preparing the material is fresh and interesting as well as the outstanding result.However, the structure and writing style of the paper is a bit difficult for readers to follow.There are information and variables that are not introduced well for full understanding.
Response: Thank the reviewer very much for the positive and constructive comments.We highly appreciate the reviewer's acknowledgment of the novelty aspect of our work.
We apologize for the structure problems of our manuscript.Actually, in this manuscript, we developed Bouligand-chiral and fibrous structured ceramic aerogels (BcF-CA) with strong, superelastic, and fatigue-resistance properties for thermal insulation applications.The whole work includes four parts: i) Fabrication and Bouligand chiral architecture of BcF-CAs; ii) Evaluation of temperature-invariant tensile performance; iii) Evaluation of temperature-invariant superelastic performance; iv) Fire resistance and thermal insulation applications.
In the first section, the inspiration and preparation process of the BcF-CAs material were elaborated in detail.Subsequently, in the second section, the finite element (FE) method was employed to thoroughly simulate the tensile properties of BcF-CAs under varying torsion angles.Additionally, the high-temperature tensile properties of the BcF-CAs were also examined.The third section focused on studying the impact of twisting angles on the compressive properties of BcF-CAs and their ability to withstand extreme environments ranging from -196 ℃ to 1200 ℃.Finally, the thermal insulation properties of BcF-CAs were evaluated.The assembled BcF-CAs exhibited exceptional high-temperature mechanical and thermal stability, indicating their potential as a desirable insulation material in various fields, including insulation layers in the aerospace industry.Furthermore, we sincerely apologize for any previous writing issues encountered in the manuscript.To address this concern, the manuscript has undergone a thorough revision by native English speakers, ensuring improved clarity and coherence.Subsequently, all authors have diligently reviewed and refined the manuscript before submission.
1.In the Abstract, could the author include the density and insulation properties of the developed material since it aims for insulation and aerospace technology?
Response: Thank the reviewer for the valuable and constructive comments, the insightful feedback is highly beneficial in refining and improving our work.
As suggested by the reviewer, we have described the density and insulation properties of the BcF-CAs in the "Abstract" in detail in the revised Manuscript.

Abstract:
"Ceramic aerogels are often used when thermal insulation materials are desired; however, they are still plagued by poor mechanical stability under significant thermal shock.Here, inspired by the dactyl clubs of mantis shrimps found in nature, which form by directed assembly into hierarchical, chiral, Bouligand (twisted plywood) structures with improved mechanical properties, we present a compositional and structural engineering strategy to develop strong, superelastic, and fatigue resistance ceramic aerogels with chiral fibers arrays resembling Bouligand architecture.Benefiting from the stress dissipation, crack torsion, and mechanical reinforcement of micro-/nanoscale Bouligand arrays, the tensile strength (170.38 MPa) of these aerogels is between one and two orders of magnitude greater than that of state-of-the-art nanofibrous aerogels.In addition, the developed aerogels feature exceedingly low density (0.223 g cm −3 ), low thermal conductivity (0.037 W m −1 K −1 ), exceptional compressive properties with rapid recovery from 80% strain, and excellent thermal stability up to 1200 ℃, making them ideal for thermal superinsulation applications.This technology holds great promise for designing and assembling aerogel architectures without the typical limitations of traditional fabrication methods."toughness, fracture resistance, and impact resistance. 1,2In addition, it has been proved that the transition from a longitudinally staggered array to a Bouligand chiral array leads to a monotonic decrease in anisotropic mechanical properties, e.g., the anisotropy ratio of Young's modulus changes from 4.25 to 1.25. 3,4Other studies have demonstrated that materials with a Bouligand array exhibit band gaps at frequencies associated with the impact stress pulse and therefore confer wave-filtering property. 5" 4. What are the advantages of combining both macro-and nano-scale components into ceramic aerogels?Where is the idea come from?
Response: We sincerely thank the reviewer for the valuable comments; the comments are constructive in improving our manuscript.

Ceramic aerogel materials exhibit the advantages of low density, low thermal conductivity, high temperature resistance, and superior thermal chemical stability 10 ;
nevertheless, the actual utilization of ceramic aerogels remains restricted owing to their inherent hardness, brittleness, and incompressibility.In order to address these limitations and enhance the mechanical properties of the ceramic aerogels, implementing ceramic nanofibers as fundamental building blocks to fabricate nanofibrous aerogels has proven to be a more efficacious approach.Such nanofibrous aerogels exhibit improved flexibility and elasticity, effectively overcoming the brittleness associated with conventional ceramic aerogels [11][12][13][14] .However, due to the macroscopic mechanical limitations of the nanocomponents, these aerogels exhibit relatively low mechanical stress (<1 MPa) and are incapable of withstanding the harsh mechanical and thermal flow shock encountered in practical applications. 14ramic microfibers, characterized by exceptional mechanical properties such as tensile strength ranging from 1.75 GPa to 3.1 GPa, [15][16][17] coupled with high-temperature stability surpassing 1200 ℃, have been extensively employed as reinforcing phases in fabricating ceramic matrix composites.9][20][21][22] Regrettably, the ultrahigh density and extreme reliance on the fibers orientations in determining material mechanics (with the highest mechanical properties observed along the axial direction of the fibers) have severely limited their further expansion in the field of hightemperature thermal insulation.

Traditional Chinese Taoism emphasizes the balance of the five elements,
particularly the concept of "Yin and Yang, rigidity and flexibility".This philosophical notion of balancing opposing forces has relevance in scientific endeavors, particularly in advancing our understanding of materials.In material science, achieving ultra-high strength necessitates the utilization of rigid components.However, a material composed solely of "rigid" elements would become brittle and heavy, impeding its practicality.Conversely, "flexibility" provides inherent lightness and pliability, but compromises macroscopic mechanics such as tensile and compressive strength if only "flexibility" elements are used.
In our work, for example, ceramic microfiber, known for its ultra-high strength, constitutes the "rigid" component.It exhibits remarkable properties, but the materials with only microfiber component fall short in brittleness and weight (2.3~2.6 g/cm 3 ).
Meanwhile, ceramic nanofibers possess inherent lightness and "flexibility", however, the materials of only nanofiber component exhibit inferior macroscopic mechanical characteristics with tensile strength below 1 MP.Therefore, by capitalizing on the dichotomy of "rigidity" and "flexibility", we integrate these two types of fibers to maximize their collective mechanical advantages.
In addition, nature is a rich source of inspiration.One example is the mantis shrimp, an appealing but deadly creature.The complex anatomy of the mantis shrimp's claws, consisting of Bouligand chiral stacking of the reinforced chitin protein fibrils, coupled with their exceptional speed, enable them to breach the defenses of even the most resilient prey such as mollusks and crabs. 23Motivated by these above, specifically, we propose an approach to utilize ceramic microfibers and nanofibers as bicomponent building blocks, combined with chiral assembly resembling the biomimetic Bouligand helix structure, to maximize the mechanical advantages of micro/nanofibers.Remarkably, we successfully developed ceramic micro/nanofiber aerogels, demonstrating a significantly reduced bulk density of only 0.215 g cm -3 while maintaining high-temperature resistance and low thermal conductivity.The exceptional properties of these aerogels position them as prime candidates in hightemperature insulation applications, encompassing aerospace, deep-sea exploration, and military industries.
5. The paragraphs after the subheading "Fabrication and Bouligand chiral architecture of BcF-CAs" look a bit long and difficult to understand.Could the author shorten them and make them clearer?In addition, it could be better to have a diagram to depict how BcF-CAs are fabricated from preparing nanofibrous membranes to the final material.

Response:
We sincerely thank the reviewer for the valuable comments, which are very helpful in improving our manuscript.
First, as suggested by the reviewer, we have carefully revised the paragraphs mentioned by the reviewer in the revised Manuscript (Page 5, line 108).

Page 5, line 108:
"The mantis shrimp, an appealing but deadly creature.The complex anatomy of the mantis shrimp's claws, consisting of Bouligand chiral stacking of the reinforced chitin protein fibrils, coupled with their exceptional speed, enable them to breach the defenses of even the most resilient prey such as mollusks and crabs.Typically, the unique Bouligand architecture is characterized by a helical array of fibril lamellae with a twisting angle, as shown in Fig. 1a.The architecture is subjected to a 180° rotation, which can be quantified by α and ϕ along the z-axis direction, where α represents the twisting angle created among two neighboring layers, while ϕ defines the distribution of twisting angle (Fig. 1b).The direction of the fibers aligned with the x-axis is denoted as ϕ = 0°, meanwhile the fibers twist counterclockwise around the z-axis."

Page 6, line 118:
"This configuration increases the crack surface area and contributes to the reorientation of the fibers in response to external stresses, such as tensile, flexural, and impact loads29.Meanwhile, the resulting modulus oscillation within the Bouligand geometry is assumed to enhance crack torsion (Supplementary Fig. S1).Incidentally, the Bouligand chiral array result in in-plane isotropic mechanical properties (Fig. 1cd and Supplementary Fig. S2), which overcome the typical limitations of materials with traditional unidirectional 3D fibrous structure."

Furthermore, we have provided additional elaboration for Fig. 1b-d to enhance comprehension. As shown in Fig.1b, the Bouligand structure exhibits an intriguing propensity to generate a substantial number of intricately interwoven microcracks characterized by twisting patterns. Meanwhile, the crack propagation occurs
.25 Consequently, the progressive development and propagation of these microcracks provide a source of energy dissipation and stress relaxation -ultimately contributing to the exceptional damage tolerance properties of the dactyl club, enabling it to withstand substantial loads without suffering catastrophic failure.

Fig. 1b Structural features and crack propagation path of typical Bouligand chiral array.
In addition, as suggested by the reviewer, we provided a detailed diagram to depict the fabrication process of the BcF-CAs in the revised Supporting Information (Supplementary Fig. 4).

Supplementary Figure 4. Schematic illustration of the manufacturing process of the
BcF-CAs.
6. Leter "ɑ" and "α" are not used consistently in the Figures and the paragraphs even though they have the same meaning.The ϕ symbol should be illustrated in Fig. 1b.

Response: We apologize for our careless mistakes. Thanks for the reviewer's correction; the comments are very helpful in improving our manuscript.
(1) In our resubmitted manuscript, we have corrected all the "ɑ" into "α".7. Temperature and the surface of the material before and after applying the butane blowtorch should be shown in Fig. 1m to support the idea that "…the BcF-CA maintained their shape and structure even under such extreme conditions…".

Response:
We sincerely thank the reviewer for the valuable comments, which are very helpful in improving our manuscript.
As suggested by the reviewer, we have provided the temperature and the surface of BcF-CA (Fig. 1m) before and after applying the butane blowtorch in the revised Supporting Information (Supplementary Fig. 14).Fig. 1m The BcF-CAs were exposed to a butane blowtorch, and no damage was observed.
Supplementary Figure 14.The BcF-CAs were exposed to a butane blowtorch before and after the treatment.
8. The author should investigate the effect of AlBSi sol on the mechanical properties of the material to choose 5 wt% as an appropriate concentration.

Response:
We greatly thank the reviewer's professional review work on our manuscript; the comments are constructive in improving our manuscript.As shown in Supplementary Fig. 9, the maximum compressive stress of aerogels stabilized at 160 kPa as the sol concentration increased to 5 wt%.Meanwhile, the tensile stress of aerogels was initially increased and then decreased (Supplementary Fig. 10); the maximum stress of 179 MPa was found at 5 wt%.We speculated that a suitable sol content would benefit the robust bonding between the fibers (Supplementary Fig. 8d and Supplementary Fig. 11).Still, excessively adhesive interfaces would be generated between the fibers and the AlBSi matrices when the sol concentration exceeded a specific value.In the latter case, too much AlBSi sol shrank during drying, but the nanofibers hardly shrank, resulting in tiny cracks induced by strong adhesion, which could accelerate the stress-failure process of BcF-CAs to some extent (Supplementary Fig. 8e-f

Page 12, line 240:
"The simulation results clearly showed that as the value of α decreased from 75° to 0°, the maximum stress (i.e., the stress at initial failure) increased (Fig. 2b).
Mechanistically, the Bouligand array enabled the fibers to reorientate in response to external stress.The majority of the fibers reorientate along the tensile direction and underwent tensile deformation owing to stretching/sliding mechanisms, while some of the other fibers rotated symmetrically in the direction away from the stress axis.
Remarkably, smaller α resulted in an increment in the proportion of reinforcing fibers in the direction of tensile loading, thereby enhancing the ductility and toughness of the BcF-CAs to prevent fracture at the macroscopic level (Fig. 2b). 36  Furthermore, the BOU-15 subjected to tensile loading in different directions were further investigated (Supplementary Fig. S18 and Fig. 2d bottom).The results indicated that no significant differences in the tensile strength of BcF-CAs were found even when the force direction was varied.That is, BcF-CAs with chiral helical fiber arrays had tensile properties that were independent of loading directions and fiber orientation, which was generally difficult to achieve in other fiber-based 3D materials.More importantly, as expected, the experimental behavior correlated well with the simulation results."Supplementary Figure 18.The BOU-15 was subjected to tensile stress in different directions (30°, 60°, 90°), where the red lines represent the directions of the force applied.
12. As the maximum tensile stress is around 170.38 MPa, however, the axis value for the stress in Fig .2g used the unit of GPa, which is difficult to see how the stress change over the temperature range.

Response:
We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
As suggested by the reviewer, we have split Fig. 2g into two figures in the revised manuscript and Supporting Information (Fig. 2g Supplementary Fig. 20) for ease of understanding.Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
(1) what is the micro-fiber content and ratio of fibers mentioned in the manuscript?

Micro-fiber content and ratio of fibers mentioned in the manuscript refers to the mass percentage of microfibers incorporated within the BcF-CAs.
(2) How to control them when preparing the material?

Page 13, line 271：
"Considering the need for ultralight BcF-CAs, the optimum content of microfibers was determined to be 15 wt %." Supplementary Figure 19.3D surface plots of the tensile stress as a function of c and density.
14.The l symbol is used in the manuscript with the meaning of the number of layers but meaning the side length in the fabrication process (supplementary file).
Response: Thanks for the reviewer's careful checks.We are sorry for our carelessness.
As suggested by the reviewer, we have corrected "l" to "h" in the revised Supporting Information.

Revised Supporting Information:
"Then the ceramic micron filament fibers were cut into staple fibers of the same length

(with a length of h) and used as the mechanically reinforced phase of the nanofibrous aerogels; the mullite nanofiber membranes were cut into squares with side length h."
15. What is the strength retention and energy loss coefficient?How to determine them?

Response:
We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
(1) What is the strength retention and energy loss coefficient?

◼ Strength Retention
The Strength Retention in Fig. 2b

of the Manuscript refers to the percentage of the strength of the material after being treated at high temperature (900-1300 ℃) for 1 h and the strength of the material itself. The value of Strength Retention corresponds to the high-temperature mechanical properties of the material. A significant Strength
Retention indicates the material has better resistance to high temperatures.

◼ Energy Loss Coefficient
In the case of viscoelastic materials, loading and unloading follow different stressstrain curves, and due to the mismatch between the loading and unloading curves, they form a closed loop, which becomes the elastic hysteresis loop (Figure 3) The area enclosed by the elastic hysteresis loop is the dissipated energy per unit volume of the material (∆U).(Here, only the loading and unloading of the material under uniaxial stress is considered).However, as shown in Figure 4 (unclosed-type hysteresis curve), it indicates that the material is poorly elastic and has undergone severe plastic deformation.(2) How to determine them?

◼ Strength Retention
To control (improve) the strength retention of the material, the following points must be considered:

i) Improve the high-temperature resistance of the building blocks; ii) Establish a stable bonding between the building blocks; and iii) design a robust microstructure inside the materials. The biomimetic Bouligand ceramic fibrous aerogels fabricated in
our manuscript fulfill these three points (high-temperature resistant micro/nanofibrous matrix elements, stable high-temperature bonding structure of AlBSi ceramics, and deformable 3D spatial network structure of Bouligand), and therefore have ideal strength retention even at extremely high temperature.

◼ Energy dissipation coefficient
To control (improve) the material with a significant energy dissipation coefficient, it is necessary to improve the energy dissipation ability of the material during the deformation process.For BcF-CAs in our manuscript, since the micro/nanofibers are interconnected to form a 3D spatial (Bouligand) chiral network, the deformation of the three-dimensional network structure absorbs a certain amount of energy during the compression process, enabling the BcF-CAs to have a high energy dissipation coefficient and thus a good deformation resistance.

Did the author demonstrate the temperature-invariant stretchability tensile properties of BcF-CAs at around -196 °C?
Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.

Moreover, temperature-invariant stretchability tensile properties of BcF-CAs in liquid
nitrogen (-196 °C) has been provided in the revised Supporting Information (Supporting Fig. 21).

Page 13, Line 282:
"The BcF-CAs had reversible strain even upon exposure to a ∼ 1200 °C butane blow torch flame or in liquid nitrogen (-196 °C), and no loss of strength or stiffness was observed (Fig. 2h and Supplementary Fig. S21).Therefore, the temperatureinvariant tensile properties of BcF-CAs from -196 °C to 1200 °C were demonstrated."Supplementary Figure 21.Temperature-invariant stretchability.Tensile and recovery processes in liquid nitrogen.

How about the tensile and compression strength of continuous blocks constructed by only microfibers and nanofibers compared to anisotropic Bouligand ceramic aerogels?
Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.

As suggested by the reviewer, we have investigated the tensile and compression strength of continuous blocks constructed by only microfibers and nanofibers and provided more corresponding explanations.
As shown in Figure 6 and Fig. 1i

, under the same volume content, the tensile strength of full-micron fiber aerogel is the highest (~1 GPa), followed by BcF-CAs, and the tensile strength of full-nanofiber aerogel is the lowest (less than 1 MPa),
which is because the strength of micron fibers is much higher than that of nanofibers and can withstand the more enormous external load, so the higher the content of micron fibers, the higher the tensile strength.Meanwhile, it can be noticed from the compressive stress-strain curves that the compressive strength of full-micron fiber aerogel (Figure 7), full-nanofiber aerogel (Figure 8), and BcF-CAs (Fig. 3a) are close to each other, but the energy dissipation rate of full-micron fiber aerogel is lower than that of full-nanofiber aerogel and BcF-CAs.In the Q15, we have demonstrated that the greater the energy dissipation, the smaller the stress wave propagation distance, which is not conducive to damage, and the material has lower brittleness and higher fracture strength.Therefore, it also indicated that the micron fiber component acts as a strong support while the nanofiber component functions as a significant stress dispersion.As shown Fig. 3a, incorporating appropriate micron fibers in the construction of BcF-CAs can significantly improve the tensile properties of the aerogels without compromising their compressive properties.

In addition, the content of micron fiber in BcF-CAs depends on the trade-off between tensile strength and bulk density.
Given that the BcF-CAs are designed primarily for aerospace applications where ultra-lightweight materials are required, choosing the appropriate micron fiber content is essential and desirable (answered in Q13).

Fig. 3a Compressive stress-strain curve of the BcF-CAs.
18.There are typos: a significant compression stain of 80% (page 14), Discussion heading should be Conclusion.

Response:
We sincerely thank the reviewer for careful reading.We feel sorry for our carelessness.
We have corrected "Discussion" to "Conclusion" in our resubmitted manuscript.Thanks for your correction.
19. Which sample is mentioned in Fig. 3a-c?
Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
The sample mentioned in Fig. 3a-c is Supplementary Fig. 15.Supplementary Figure 15.The anisotropic mechanical properties of the BcF-CAs.20.Could the author double-check Young's modulus in Fig. 3c?What is the strain used to determine the stress in that figure ?Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
(1) Could the author double-check Young's modulus in Fig. 3c?We apologize for the mistakes in the Manuscript.As suggested by the reviewer, we have double-checked and revised Young's modulus in Fig. 3c and Supplementary Fig. 25 in the revised manuscript and Supporting Information.In addition, we also have carefully checked similar deficiencies throughout the manuscript.

Supplementary Figure 25. Young's modulus, energy loss coefficient, and maximum stress as a function of compression cycles
(2) What is the strain used to determine the stress in that figure ?We apologize for not describing Fig. 3c enough, which caused the reviewer to be confused.
The strain that determines the stress in Fig. 3c is from the cyclic test in Fig. 3b.
The data provided in Fig. 3c is a supplement to the compressive cycling tests in Fig. 3b.

Does changing the torsion angle affect the density and thermal conductivity of the material?
Response: We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
(1)Theoretically, varying the twist angle does not affect the density of a material.

The density is a function of the volume and weight of the material. However, changing
the twist angle does not change the volume and weight, so the density of the material is not varied.
(2)As shown in Fig. 4b in the manuscript, the thermal conductivity increased with the density of the BcF-CAs.However, we have known that the torsion angle did not affect the density of the BcF-CAs.Consequently, the thermal conductivity is unchanged.

Fig. 4b
Thermal conductivity of the BcF-CAs as a function of density.

Please provide reference and thermal conductivity (if possible) for Supplementary
Table 1.

Response:
We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.In the present manuscript, the authors demonstrate the "Bouligand" (or twisted) structure that is seen in nature is suitable for strengthening fiber-based porous materials.

Materials
The idea is interesting and rather easy to realize, and the tensile strength of the freezedried aerogel-like material is impressively high.In my opinion, however, in addition to the enhancement of mechanical strength, the authors have to report the thermal conductivity at temperatures since the authors mention applications in the aerospace industry.To gain enough novelty to be accepted by Nature Communications, this reviewer requests to clarify this point (comment 5 below), together with some other revisions.

Response:
We sincerely thank the reviewer for the positive and constructive comments.
1.The preparation process should be briefly written in the main text so that readers can understand what the materials are like.Especially for understanding how the angles were made between the layers and what is the AlBSi sol and its role.

Response:
We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
We apologize for not providing details about the fabrication process of BcF-CA, which makes the reviewers puzzled.In fact, we have added a schematic illustration of the manufacturing process in response to the Q5 of the first reviewer (Supplementary Fig. 4).Additionally, regarding the other concerns of the reviewer, we have provided detailed responses as follow: (1) Preparation process and what is the AlBSi sol and its role.

Page 7, Line 148：
"In the proof-of-concept study, mullite nanofibers and Al2O3 macrofibers were carefully selected as the sample materials due to their superior thermal stability. 1Fig. 1e and Supplementary Fig. S4 present the fabrication process of BcF-CAs.The fabrication strategy started with electrospinning mullite/poly (ethylene oxide) (PEO) sol to produce flexible mullite nanofibers (Supplementary Method).The obtained mullite nanofiber had a diameter of 310-420 nm (Supplementary Fig. S5) and a tensile strength of 0.47 MPa (5% strain) (Supplementary Fig. S6).The Al2O3 microfibers (Supplementary Fig. S7) possessed a fiber diameter of ~7 μm and displayed an impressive tensile strength of 2.1 GPa.Then, the ceramic micro/nanofibers were subjected to immersion in AlBSi sol for 2 h.Notably, AlBSi is known for its remarkable thermal stability and mechanical properties, making it a "ceramic glue" for bonding adjacent fibers.This cross-linking method relied on the formation of silicate bonds (X-O-Si), achieved through the calcination of silica nanofibers in the presence of oxygen. 1 Finally, the immersed macrofibers and nanofibers were arranged layer-bylayer with a specific helical angle (15°, 30°, 70°, 0°) and freeze-dried for ∼48 h to obtain unbonded BcF-CAs.After annealing in a muffle furnace (900 °C for 1 h in flowing air), bonded BcF-CAs were obtained." (2) About how the angles were made between the layers: We developed a "compass" (just like Fig. 9b) guided layer-by-layer method for preparing spiral laminated samples with different interlayer twist angles.During the layer-by-layer lay-up process, each rectangular sheet was positioned at the diagonal incremental markers at the corners of two diameters opposite each other.Subsequently, the sheets were stacked layer-by-layer according to the variation in helix angle between each consecutive layer (Figure 9b).Apparently, parallel (0°/180°) laminates were easy to construct.However, helical stacked structure samples with other twist angles need more precise positioning during lay-up preparation.By implementing this preparation method, we ensure positioning inaccuracy remains below 1.5°.
However, since this method is only available in the laboratory, we intend to develop an automated lay-up machine with a motorized rotating table, offering the flexibility to adjust the twist angle based on sample requirements (Figure 9a).

Figure 9. (a) An automated lay-up machine with a motorized rotating table. (b)
The compass that guided the layer-by-layer method in our manuscript.
2. Why were the two precursors of aluminum chloride and aluminum isopropoxide used?
Response: We sincerely thank the reviewer for the valuable comments, which are very helpful in improving our manuscript.

Response:
We thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
We sincerely apologize for our carelessness.As pointed out by the reviewer, the pore size of the BcF-CAs indeed be classified as macropores (> 50 nm).We have appropriately revised the manuscript (Page 17, Line 375).Additionally, we have thoroughly double-checked the manuscript to avoid similar problems.Once again, we thank the reviewers for their careful suggestions.

Page 17, Line 375:
"For the BcF-CAs, λconv was severely restricted due to the blocking of air in individual macropores." 5. The fatal drawback of this manuscript is there is no report on the thermal conductivity at elevated temperatures.The thermal conductivity values at room temperature are reported, but the novelty of this material is its mechanical strength at high temperatures like 1200 deg C, and the authors mention the use as an insulation layer in the aerospace industry (Fig. 4a).Therefore, it is needed to show the thermal conductivity values at such high temperatures, without which readers cannot judge this material is suitable for such applications or not.In addition, since the material loses strength above 1300 deg C, it must be used at 1200 deg C or lower.The authors should verify that the 1200 deg C is high enough for those applications or not.I am not sure how high the temperature is in the exhaust of the rocket engines.Also, the high-temperature thermal conductivity must be lower compared to other insulation materials, and the authors have to prove the value is low enough for such applications.

Response:
We sincerely thank the reviewer for the valuable comments, the comments are very helpful in improving our manuscript.
(1) The authors should verify that the 1200 deg C is high enough for those applications or not.

Before answering the reviewer's question, I would like to introduce the cooling technologies of the rocket engine and nozzle.
The engine, often referred to as the "heart" of the rocket, serves as the primary power source that propels the rocket into space.Through a series of cycles, fuel is burned in the main combustion chamber, resulting in high temperatures, volumetric expansion, and high-velocity jetting that ultimately generates thrust.Remarkably, inside the main combustion chamber, the gas temperature can reach staggering levels, as high as 3500 K (Figure 11), which exceeds the melting point of most materials and is approximately half the temperature of the scorching surface of the sun.
To ensure that the engine operates normally and efficiently for a relatively long time, the engine generally possesses a cooling system.Typically, it involves various cooling methods, including but not limited to:

① Thickened Metal Walls for Heat Dissipation and Cooling
Thickening the combustion chamber and nozzle walls with metal (titanium or nickelbased alloys) allows the engine to absorb more heat and dissipate it immediately before it reaches melting point.However, this method requires significant material consumption, increased strength requirements due to the highly uneven distribution of material thicknesses, and, most fatally, redundant mass, which is a considerable challenge and potential detriment to the overall efficiency of the engine.
According to the Tsiolkovsky rocket equation: Where ∆ is the increased velocity of the rocket,   is the emission velocity, i.e., the velocity at which the rocket fuel and oxidizer are ejected, and m0 is the initial mass of the rocket, m1 is the sum of the pure mass of the rocket after acceleration.This equation shows that the dry mass ratio (  0  1 ) directly determines the velocity increment of the rocket.Therefore, to avoid excessive mass of the equipment, only the combustion chamber is considered using this type of heat dissipation.

② Adjusting the Fuel Mixing Ratio
The temperatures at which fuel burns depend on the fuel-to-oxidizer ratio.The ratio that allows the fuel and oxidizer to react and burn fully is based on the "stoichiometric ratio" of the reaction equation.However, complete combustion results in high temperatures, which places greater demands on the cooling system of the engine.Therefore, to reduce the temperatures in the pre-combustion chamber and gas generator, similar mixing ratio variations are applied to the pre-combustion in the engine gas generator cycle, and the staged combustion cycle and the full-flow staged combustion cycle are designed with an oxygen-rich pre-combustion chamber and a combustion-rich pre-combustion chamber.For example, the SSME RS-25 engine turbopump is designed with a mixing ratio deviation for rich combustion, and the Soviet NK-33 closed-cycle engine turbopump is designed with oxygen enrichment.

③ Ablative Cooling
Ablative cooling is a method of reducing nozzle temperature by vaporizing the outer material and dissipating the heat.Typically, an ablative cooling nozzle consists of a high-melting-point carbon composite material, which is ablated at high temperatures to manage thermal energy effectively.Uniquely, this cooling approach eliminates the requirement for additional engine components and strictly follows the physicochemical principles of self-regulation of the external environment temperature feedback, which makes the cooling effect better and more reliable.

④ Fuel/Cold Air Cooling
Fuel/cold air cooling is achieved by injecting a layer of liquid (cold fuel)/cold air between the combustion chamber wall and the nozzle wall to isolate the two hightemperature objects, thereby preventing excessive temperatures and thermal shock.To increase the contact area for better thermal transfer, the engine nozzle wall is filled with numerous tiny fuel/cold gas channels.The mainstream manufacturing method currently used is to cut out the corresponding tubes in the nozzle wall and seal them with metals such as copper and nickel alloys with high thermal conductivity to form fuel channels.The above cooling methods reduce the temperatures of the engine and nozzles, and they are generally about 1000 °C .In fact, reaching the ideal temperature will inevitably increase the weight of the machine and reduce the efficiency of fuel use, which is not conducive to the flight of the aircraft and is a waste of energy.Therefore, an additional insulation layer is required.BcF-CAs have the advantages of hightemperature resistance and low density, which makes them an ideal insulation layer (Figure 12).The insulation layer refers to the thermal insulation between the hightemperature resistant (titanium, nickel-based alloy, fiber-reinforced ceramic-based composite) inside the nozzle and the non-high-temperature resistant outside, and it can be used as a backer for the thermal shock resistance.In other words, our material (BcF-CAs) can satisfy the use of high-temperature hot-end parts, which not only leads to a significant weight reduction but also saves cold air, improving the total pressure ratio, increasing the operating temperature of 400 -500 °C based on the traditional cooling method, and reducing the weight of the structure by 50% -70%, making it the critical thermal structure material for the upgrading of aircraft engines.
(2) It is needed to show the thermal conductivity values at such high temperatures, without which readers cannot judge this material is suitable for such applications or not.
In addition, since the material loses strength above 1300 deg C, it must be used at 1200 deg C or lower.Also, the high-temperature thermal conductivity must be lower compared to other insulation materials, and the authors have to prove the value is low enough for such applications.
Given the above discussion, it is known that the temperature of space engines such as rockets is around 1000 °C after cooling through a series of cooling systems, in which case BcF-CAs can be applied.Then, as suggested by the reviewer, we tested the high-temperature thermal conductivity λt of BcF-CAs.As shown in Figure 13, the λt of the BcF-CAs was only 0.076 m W m -1 K -1 at 300°C.Meanwhile, λt increased with the higher temperature, reaching 0.225 m W m -1 K -1 at 1200 °C.This is due to the following two reasons: i.
The current predominant method for high temperature thermal conductivity testing is the Hot Wire Method 26,30,[38][39] , but it is limited to a temperature range of less than 1000 °C.Therefore, we choose the Protective Hot Plate Method for testing.However, this method inevitably has some deviations from the Hot Wire Method, resulting in our data being larger than the actual values.
ii.The λrad is usually negligible at temperatures below 100 °C, but becomes the main source for the thermal conductivity of BcF-CAs at high temperatures.
λ rad can be calculated by where λ rad is the conductivity of the thermal radiation, n is the refractive index of the heat transfer medium,  is the Stefan-Boltzmann constant, e is the absorption coefficient (mass attenuation coefficient), and  is the density of the heat transfer medium.From Eq. ( 7), the λ rad scaled linearly as λ rad ~   3 , represents a large λ rad under high temperature conditions.These results indicate the exceptional thermal insulation capabilities of BcF-CAs even under extreme conditions and accentuate their potential for application as highly efficient thermal insulating materials.Notably, despite the rather high λt measured on our BcF-CAs using Protective Hot Plate Method, it is comparable to the thermal conductivity of current thermal insulation materials (Fig. 14). 26,38 herefore, in summary, our materials are fully suitable as potential candidates for aerospace insulation materials.
We tried our best to improve the manuscript and made some changes marked in red in the revised paper, which will not influence the content and framework of the paper.We appreciate the Editors/Reviewers' warm work earnestly and hope the correction will be approved.Once again, thank you very much for your comments and suggestions.

2 . 3 .
In the Introduction, data should be added to support opinions, e.g., "Ceramic aerogels exhibit an extraordinary combination of low thermal conductivity, high stability in extreme environments,…", "…It has been shown that the transition from longitudinally staggered array to Bouligand chiral array leads to a monotonic decrease in anisotropic mechanical properties…" Response: We sincerely thank the reviewer for the valuable comments; the comments are constructive in improving our manuscript.As suggested by the reviewer, we have added data mentioned by the reviewer to support opinions in the manuscript (page 3, line 45; page 3, line 87).Page 3, line 45, "Ceramic aerogels exhibit an extraordinary combination of low thermal conductivity(from 0.012 W m −1 K −1 to 0.033 W m −1 K −1 in the air ), good stability in extreme environments (-196 ~ 1200 ℃),…", Page 4, line 87, "…it has been shown that the transition from a longitudinally staggered array to a Bouligand chiral array leads to a monotonic decrease in anisotropic mechanical properties, i.e., the anisotropy ratio of Young's modulus changes from 4.25 to 1.25…"Moreover, we have double-checked similar deficiencies throughout the manuscript.Could the author introduce some previous research working with the Bouligand structure to illustrate how good it is?Response: We sincerely thank the reviewer for the valuable comments; the comments are constructive in improving our manuscript.As suggested by the reviewer, we have provided previous research on Bouligand structure in the revised Manuscript (Page 4, line 81).Page 4, line 81: "Previous studies have shown that the Bouligand chiral conformation provides superior energy absorption, effective stress transfer, and the ability to inhibit crack propagation by twisting and reorienting ordered nanofibers under external loads, thereby endowing materials with outstanding mechanical properties including

(( 2 )
Page5, line 114; Page 12, line 245; Page 14, line 297; Page 14, line 301).Moreover, we have carefully double-checked similar deficiencies throughout the manuscript.Page 5, line 114, "Specifically, the architecture is subjected to a 180° rotation, which can be quantified by α and ϕ along the z-axis direction, where α represents torsion angle created among two neighboring layers, and ϕ defines the distribution of the twisting angle along the crack propagation path (Fig. 1b)."Page 12, line 245, Remarkably, smaller α resulted in an increment in the proportion of reinforcing fibers in the direction of tensile loading, thereby enhancing the ductility and toughness of the BcF-CAs to prevent fracture at the macroscopic level (Fig. 2b)."Page 14, line 297, "The results indicated that the small α (15°) enhanced the ability to tolerate large deformations."Page 14, line 301, "…arrangement with a small α induced a gradual variation in in-plane stiffness and was expected to reduce interlaminar shear stresses -a critical factor contributing to dislamination…" As suggested by the reviewer, we have illustrated the ϕ symbol in Fig. 1b in the revised Manuscript.Thanks for the reviewer's correction.

). Supplementary Figure 8 .
The surface morphological SEM images of (a) AlBSi-0.5 wt%, (b) AlBSi-2 wt%, (c) AlBSi-4 wt%, (d) AlBSi-5 wt%, (e-f) AlBS-8 wt%.Supplementary Figure 11.The robust bonding between macro-fibers and nanofiberswith AlBSi-5 wt%.9.Could the author show how to determine the thickness of the bonding layer?Response: Thank the reviewer for the constructive comments.We apologize for not clearly representing the factors that affect the thickness of the AlBSi bonded layer in the Manuscript, which confuses the reviewer about the design principle.We determined the thickness of the bonded layer by combining SEM and TEM images.The process of fabricating BcF-CAs, as described in the Q15, involves immersing the micronanofibers in AlBSi sol to deposit a bonding layer on their surface for stable bonding between the adjacent fibers.The thickness of bonding layers mainly depend on the concentration of AlBSi sol and the immersion time.In the Q8, we observed the microscopic morphology of fibers with different sol concentrations, and itcan be easily found from the SEM images that the higher the sol concentration, the larger the thickness of the bonding layer on the surface of the fibers.In addition, we have supplemented the TEM images of fibers with different immersion times to further observe the variation in the thickness of the bonding layer.As shown in Figure1, the thickness of the bonded layer on the fiber surface increased with the impregnation time.

Figure 2 .
Figure 2. HRTEM-EDS images of the ceramic bonding layer on the surface of the fiber.10.Could the author give more description for Fig. 2b?Response: Thank the reviewer for the constructive comments.We apologize for not presenting the experimental details of Fig. 2b.In the revised Manuscript, we have provided corresponding details and discussions (Page 12, line 240).

" 11 .
Fig. 11d does not look quite obvious.Response: Thank the reviewer for the constructive comments.May I understand what the reviewer wants to express is the problem of Fig. 2d?We apologize for not presenting the details of Fig. 2d.First, as suggested by the reviewer, we have revised Fig. 2d in the revised Manuscript.In addition, we have provided more detailed descriptions of Fig. 2d to make it easier for the reader to understand.(Page 12, line 260).

Fig. 2g
Fig. 2g Strength retention rate and Young's modulus as a function of temperature

Figure 3 .
Figure 3. Mechanical energy dissipated by loading and unloading aerogel.

Figure 5 .
Figure 5. Aerogels are susceptible to fatigue under multiple cycles of large deformation.

Figure 11 .
Figure 11.The temperature of the combustion chamber has far exceeded the melting point of the inconel alloy, reaching more than half the temperature of the sun.

Figure 12 .
Figure 12.The diagram of fuel/cold air cooling and the position of the insulating layer.

Figure 13 .
Figure 13.Thermal conductivity of BcF-CAs as a function of temperature (steady-state thermal measurement in air).